In optical communications systems, lasers are used in optical transmitters and optical transceivers to convert electrical data signals into optical data signals, which are then transmitted over an optical waveguide, typically an optical fiber, to some intended destination, such as to an optical receiver or transceiver. Parallel optical transmitters and transceivers include multiple optical transmit channels, each of which has a respective laser for generating a respective optical data signal to be transmitted over the respective optical channel. In many parallel optical transmitters and transceivers, the output power level of at least one of the lasers is monitored by an optical output power feedback monitoring system that adjusts the modulation and/or bias currents of the lasers such that the average output power levels of the lasers are maintained at a desired or required level. Typically, the adjustments are made to cause the average output power levels of the lasers to be maintained at a predetermined, substantially constant level.
It is common practice in the optical communications industry to use a monitor photodiode to detect light output from a rear portion of the transmitter laser (or a portion of the output power reflected back through optical lenses) and to use this optical feedback to measure and control the average optical output power level of a laser. In general, the average transmitted output power level, PAVG, of the laser can be controlled by controlling the bias current, IBIAS, of the laser. Thus, if the optical feedback indicates that PAVG has fallen below the required level, increasing IBIAS by an appropriate amount will raise PAVG to the required level. Similarly, if the optical feedback indicates that PAVG has risen above the required level, decreasing IBIAS by an appropriate amount will lower PAVG to the required level.
FIG. 1 illustrates a block diagram of a typical configuration of an optical output power feedback monitoring system of a known parallel optical transmitter or transceiver. The feedback monitoring system includes n feedback control loops 21-2n, where n is the number of optical transmit channels in the optical transmitter or transceiver. The feedback control loops 21-2n are identical in configuration. The manner in which the optical output power feedback monitoring system operates will be described with reference to feedback control loop 21. The feedback control loop 21 includes a laser diode driver circuit 3, a current source 4, a laser diode 5, a monitor photodiode 6, and an analog-to-digital converter (ADC) 7. The laser diode 5 is modulated with an electrical data signal (not shown) to cause the laser diode 5 to produce an optical data signal 8. The optical data signal 8 is optically coupled via coupling optics 9 into an end of an optical fiber 11. The coupling optics 9 optically couple a small portion 12 of the optical data signal 8 onto the monitor photodiode 6.
The monitor photodiode 6 converts the portion 12 of the optical data signal 8 received into an analog electrical signal 13. The ADC 7 converts the analog electrical signal into a multi-bit digital feedback signal 14. The digital feedback signal 14 is fed back to the laser diode driver circuit 3. The control circuit 3 compares the digital feedback signal 14 to a pre-selected digital reference signal 15 and output a drive signal 16. The drive signal 16 drives the current source 4, which causes the bias current of the laser diode 5 to be varied, thereby causing the average output power level of the laser diode 5 to be maintained at a predetermined, substantially constant level.
In some cases, a single feedback control loop 21 is used to monitor the output power level of one of the laser diodes 5, in which case all of the bias currents of all of the laser diodes 5 are adjusted by the same amount based on the optical feedback from one of the laser diodes 5. In other cases, each optical channel of the parallel optical transmitter or transceiver has a respective feedback control loop 21, as depicted in FIG. 1.
One of the disadvantages of the optical output power level feedback monitoring systems of the type shown in FIG. 1 is that the photodiodes 6 can receive optical crosstalk in the form of portions of the laser light produced by the laser diodes 5 of one or more adjacent and non-adjacent optical channels. This optical crosstalk can result in errors in the amounts by which the bias currents of the laser diodes 5 are adjusted, which, in turn, can result in a failure to maintain the average optical output power levels of the laser diodes 5 at proper levels. This is particularly true when one or more of the laser diodes 5 is enabled or disabled. For example, if the laser diode 5 of feedback control loop 2n is disabled, the photodiode 6 of feedback control loop 2n-1 will receive less light due to the absence of optical crosstalk from the laser diode 5 of feedback control loop 2n. As a result, the digital feedback signal 14 will have an artificially low value, which will result in the drive signal 16 being too great. Consequently, the current source 4 of feedback control loop 2n-1 will cause the optical output power level of its laser diode 5 to be increased more than necessary, which can result in degradation in link performance, eye safety issues, and other problems.
Accordingly, a need exists for a method and apparatus that enable the absence and presence of optical crosstalk to be compensated for in optical output power feedback monitoring systems used in parallel optical transmitters and receivers.